This invention relates to a method of bonding aromatic polyamide fibers to rubber compounds.
Industrial rubber products such as automobile tires, rubber hoses, power transmission belts and conveyor belts are normally reinforced with fiber materials. Synthetic fibers are widely used as such reinforcing fiber materials since they are in general superior to natural fibers such as cotton, wool or hemp in properties such as strength, modulus of elasticity, resistance to friction and dimensional stability when being exposed to water or heat. In recent years, polyamide fibers including aliphatic polyamide fibers and aromatic polyamide fibers and polyester fibers are especially preferred as reinforcements among a variety of synthetic fibers.
Meanwhile, a variety of methods of binding fibers to rubber compounds or mixes are known, among which there is a well known method wherein fibers are treated with so-called RFL solutions, namely aqueous mixtures of resorcinol/formalin resins and rubber latices, and placed in contact with rubber compounds, and then the rubber compounds are vulcanized together with the fibers. With this method, adhesion is attained to a degree between synthetic fibers and rubber compounds. However, adhesion between synthetic fibers and rubber compounds is an important factor in determining the properties of the industrial rubber products. In this regard, since synthetic fibers are poorly wettable to RFL solutions, the adhesion attained is still unsatisfactory.
Therefore, a number of methods have heretofore been proposed wherein a variety of modified RFL solutions are employed to improve adhesion between synthetic fibers and rubber compounds. For instance, a method is disclosed in Japanese Patent Laid-open No. 49-96048 in which an RFL solution is used which contains a chlorohydrin rubber latex and a chloroprene rubber latex together with resorcinol/formalin resin for bonding polyamide fibers to chloroprene rubber mixes.
A further method is also disclosed in Japanese Patent Laid-open No. 59-89375 wherein an RFL solution is used which is composed of an aqueous mixture of a chloroprene/dichlorobutadiene copolymer latex and resorcinol/formalin resin.
On the other side, high saturation or complete saturation rubbers such as high saturation nitrile rubber, ethylene/propylene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, epichlorohydrin rubber or fluorocarbon rubber have come to be used increasingly in various industrial fields in recent years. These rubbers are more poorly bonded to synthetic fibers.
Thus, a still further method has been proposed, as disclosed in Japanese Patent Laid-open No. 61-207442, to bond high saturation hydrocarbon rubbers such as hydrogenated nitrile rubbers to fibers wherein an RFL solution is employed which is composed of aqueous mixtures of halogen-containing polymers such as epichlorohydrin rubber, chloroprene rubber, chlorosulfonated polyethylene rubber or chlorinated polyethylene rubber, together with resorcinol/formalin resin.
However, aromatic polyamide fibers are very poorly wettable to RFL solutions due to their more inactive surface than those of aliphatic polyamide fibers or polyester fibers. Accordingly, vulcanized composite products of aromatic polyamide fibers and rubbers produced by such known methods as above do not stand uses where a large shearing force is generated between the rubber and fibers by, for example, bending, compression or elongation under high temperature conditions, since the adhesion therebetween is insufficient and leads to separation failure at the interface between the fibers and rubbers. Thus, the composite products are of short lift and fail to make the best use of excellent properties that the aromatic polyamide fibers possess.